This invention relates to gaging apparatus for determining deviations of a material property and, more particularly, to an improved gaging apparatus and method of the non-contacting type wherein a beam of radiation is passed through the material to be measured and then detected to determine properties of the material.
Thickness measuring gages of the non-contacting type are conventionally used to determine deviations in thickness of certain materials, for example metals such as steel or aluminum processed at rolling mills. The gage typically includes a source of penetrative radiation, such as X-rays, which directs radiation toward a detector spaced from the source. The space between the source and the detector is defined as an inspection field and the material to be gaged, usually a strip or sheet of metal, is disposed in the inspection field and causes a degree of attenuation of the radiation. Variations in the attenuation caused by the material being measured are indicative of variations in the thickness of the material. However, a number of complicating factors exist, since the measured attenuation is also a function, inter alia, of the material's composition and the intensity and wavelength of the radiation. Since the effect of these various factors upon the measurements are not generally linear, most system designs strive to keep as many factors as possible within known ranges to minimize errors.
Available radiation gages often cover an overall range of measurement divided into multiple subranges. Each interval is generally determined by presetting the level of either the radiation source or the detector, or both. So-called "standard" pieces of metal to be measured are temporarily positioned in the inspection field. The standards are of precisely known thickness and can accordingly be used to calibrate the gage for measurement of a material expected to have approximately the same thickness as the standard. In some equipments, means are provided for inserting a number of different standards of different known thickness into the inspection field, this being done with a subsystem known as an automatic standards magazine. In one type of operation, the standards are only utilized beforehand during a calibration procedure, such as to determine the null setting of a detector meter or to calibrate full scale deflections on the meter. In such case, standards are not present in the inspection field during the measurement phase of operation. In another type of operation, known as a "complementary" technique, standards of known thickness are inserted in the inspection field of conjunction with the material being measured such that the thickness of the standards plus the material being measured yields an attenuation that is expected to substantially equal one of a number of precalibrated points within the total range of the gage.
Existing systems are found to suffer one or more operational disadvantages. The number of available standards is limited and the expected thickness of the material to be measured (i.e., the "nominal" thickness) does not necessarily correspond to the thickness of an available standard. Interpolation is possible, but is further complicated by the fact that drifts in the radiation source or detector over relatively short periods of time can cause inaccuracies. Also, variations in the measured attenuation are generally not linear. Accordingly, a certain degree of guesswork, or simplifying assumptions, are often introduced during calibration or recalibration. A further factor, not yet discussed, is that the material to be measured may likely be an alloy having a composition which is different than the available full set of standards, and it is typically impractical to obtain a full range of sample thicknesses for each alloy that might be encountered. Since absorption of radiation varies substantially with the material's composition, it is necessary to correct for this variation in calibrating the gage. To illustrate a typical prior art technique, assume that the available standards are formed of a "pure" metal and the material to be measured is an alloy of that metal which exhibits a generally greater degree of absorption of radiation than the pure metal. The term Absorption Index ("AI") is defined as follows: ##EQU1## where the apparent thickness is the thickness of a pure metal standard which would result in the same detector measurement as alloy material having the nominal thickness. In this example, the AI is positive since the apparent thickness of a pure metal standard is greater than that of the nominal thickness of alloy. (In other words, since the alloy is more absorbing of radiation, a greater thickness of pure metal would be need to obtain the same degree of attenution.) In the prior art, the absorption index for a particular alloy to be measured is typically determined by temporarily positioning a sample of the alloy, of known thickness, in the inspection field and determining the AI for the alloy by comparing the measurement with the measurement taken on a pure metal standard of the same thickness. The determined AI might then be used in calibrating the gage. Unfortunately, the absorption index is not constant, but varies, inter alia, with the thickness of the materials being compared. Accordingly, if the nominal thickness of the alloy to be measured is different than the thickness of available alloy samples, the determined AI may be inaccurate and introduce error. Alloy samples of many different thicknesses may not be conveniently available, and a problem exists in accurately gaging thicknesses of alloy materials to be measured, especially when the exact absorption characteristics of the alloy are not known beforehand.
It is one object of the present invention to provide solution to the prior art problems as set forth and to devise a novel gaging apparatus and method which can be accurately and conveniently calibrated and subsequently determines material thicknesses automatically and without guesswork.